Fuel injection has long been used in engine applications. Port injection systems inject the fuel directly into the intake port of the engine (e.g., as the intake valve opens). Direct injection systems, on the other hand, inject the fuel directly into the cylinder. Issues which are often confronted in fuel injector design are the amount of force required to open the fuel injector for the discharging of fuel and the time required to completely open/close the fuel injector.
Port injection systems allow for the use of reduced forces for opening the fuel injector. Specifically, since the injection from the fuel injector is into a relatively low pressure environment (e.g., since the engine piston is on its intake stroke), a return spring of reduced capacity may be used to close the fuel injector (e.g., less spring force is required to close the fuel injector). Direct injection systems which allow for the use of reduced forces for opening the fuel injector do so by opening the fuel injector when the piston is at or near its bottom-dead-center position when the pressure in the cylinder is also relatively low.
The above-described port and direct fuel injection systems which allow for reduced actuation forces for the fuel injector suffer from reduced volumetric efficiencies. One way to enhance the volumetric efficiency is to inject the fuel as the piston approaches its top-dead-center position. However, since the cylinder pressure in this case exceeds both the pressure in a port injection system as well as the pressure in a direct injection system which injects fuel with the piston at the bottom-dead-center position, more force is required to close the fuel injector. In the case where the "closing force" is provided by a return spring, this means that the spring must have an increased capacity (e.g., increased force-generating capabilities). This in turn requires the use of larger actuation forces to open the fuel injector for injection. Consequently, direct injection systems which inject with the piston at or near its top-dead-center piston typically require larger actuation forces (e.g., at least about 50 pounds-force) to open the injector in comparison to direct injection systems which inject with the position at or near its bottom-dead center position.